The present invention relates to a high speed electroetching method to provide permanent domain refinement for electrical steels to yield improved magnetic properties.
The core loss properties of electrical steel may be improved by metallurgical means such as better orientation, thinner gauge, higher volume resistivity and smaller secondary grain sizes. Further improvements in core loss are obtainable by non-metallurgical means which reduce the wall spacing of the 180 degree magnetic domains. High-stress secondary coatings impart tension which decreases the width of the domain. The domain refinement of most interest has been the creation of a substruture which regulates the domain wall spacing. Various means to subdivide the domains have included: (1) narrow grooves or scratches by mechanical means such as shotpeening, cutters or knives (2) high energy irradiation such as a laser beam, radio frequency induction or electron beam and (3) chemical means to act as a grain growth inhibitor diffused or impregnated onto the steel surface such as a slurry or solution of sulfide or nitride compounds. All of these means are generally discussed in U.S. Pat. No. 3,990,923. Grooves or scratches have been applied to electrical steels resulting in internal stresses and plastic deformation which subdivides the large domains typically found in large grains into regions of smaller domain sizes. U.S. Pat. No. 3,647,575 uses a knife, metal brush or abrasive powder under pressure to form grooves less than 40.times.103 mm deep and spaced between 0.1 and 1 mm. The grooves may be transverse to the rolling direction and are applied subsequent to the final anneal. A stress relief anneal of about 700.degree.C. is optional. The Mar. 1979, No. 2, Vol. MAG-15, pages 972-981, from IEEE TRANSACTIONS OF MAGNETICS discussed the effects of scratching on grain oriented electrical steel in an article entitled "Effects of Scratching on Losses in 3-Percent Si-Fe Single Crystals with Orientation near (110) [001]" by Tadao Nozawa et al. The optimum spacing between scratches was from 1.25 mm to less than 5 mm. The benefits of tensile stresses were noted. All of the samples were chemically and mechanically polished prior to scratching to obtain bare, uniformly thick and smooth surfaces for good domain observations using the scanning electron microscope. Scratching was conducted after the final anneal using a ball-point pen loaded with a 300 gram weight to produce a groove which was about 0.1 mm wide and 1 mm deep.
U.S. Pat. No. 4,123,337 improved the surface insulation of electrical steels having an insulative coating by electrochemical treatment to remove metallic particles which protrude above the insulative coating.
U.S. Pat. No. 3,644,185 eliminated large surface peaks by electropolishing while avoiding any significant change in average surface roughness.
The prior art has not optimized the groove depth for permanent domain refinement in a manner which avoids damage to the surface conditions. The prior art has been limited regarding line speed to produce the series of grooves for domain refinement. By using a process which combines grooving techniques with an electrolytic etch, the problems with depth control and surface damage may be overcome. The line speed for this combined process becomes commercially attractive. The present invention provides grooves or rows of pits of sufficient depth to penetrate the coating thickness and then electroetches the exposed base metal to a critical depth to obtain permanent domain refinement.